20.3 Re-evaluating forms

If you change
val
to some other number, and want to know the sum of three
val
s again, you can avoid re-typing the form which computes it. To see how this is done, follow the instructions below.

Type
(setq val 1)
.

The variable
val
is now set to 1.

Press
Meta+P
or choose
History > Previous
or click

.
PROMPT >
(setq val 1)

The form you previously typed appears at the prompt. At this point, you could edit this form and press
Return
to evaluate the edited form. For the moment, just carry on with the next instruction.

Press
Meta+P
again, and then press
Return
.

PROMPT >
(+ val val val)

3

PROMPT >

Pressing
Meta+P
a second time displayed the second to last form that you evaluated. This time, pressing
Return
immediately afterwards simply re-evaluates the form. Note that you could have edited the recalled form before evaluating it. You can use
Meta+P
repeatedly, recalling any form that you have evaluated in the current session.

This time the form evaluates to the number
3
, because the value of
val
was changed in the interim.